The invention applies to the manufacture of flat tension mask color cathode ray tubes (CRTs). More specifically, the invention provides interrogation means for achieving registration of the aperture patterns of flat tension shadow masks and related cathodoluminescent screens. The interrogation means of the present invention could also be used to inspect these light-transmitting work pieces in preparation for subsequent operations in the CRT manufacturing process
In particular, the invention relates to a portion of the process steps employed in the manufacture of the front glass panel assembly of a flat tension mask color cathode ray tube. The complete front glass panel assembly includes a glass front panel, a shadow mask support structure on the inner surface of the front glass panel and a tensed foil shadow mask affixed to the support structure.
In this specification, the terms "grille" and "screen" are used, and apply generally to the pattern on the inner surface of the front panel. The grille, also known as the black surround, or black matrix, is widely used to enhance contrast. It is applied to the panel first. It comprises a dark coating on the panel in which holes are formed to permit passage of light, and over which the respective colored-light-emitting phosphors are deposited to form the screen.
The holes in the grille must register with the columns, or "beamlets", of electrons passed by the holes or slots in the shadow mask. This is the primary registration requirement in a grille-equipped tube; the phosphor deposits may overlap the grille holes, hence their registration requirements are less precise.
In tubes without a grille, on the other hand, it is the phosphor deposits which must register with the columns of electrons. The word "screen", or "screened" when used in the context of the faceplate, therefor includes the grille where a grille is employed, as well as the phosphor deposits when there is no grille.
Historically, color cathode ray tubes have been manufactured by requiring that a shadow mask dedicated to a particular panel follow the panel through various states of the manufacturing process. Such a procedure is more complex than might be obvious; a complex conveyor system is needed to maintain the marriage of each mask assembly to its associated panel throughout the manufacturing process. In several stages of the process the panel must be separated from the mask and the mating shadow mask cataloged for later reunion with its panel mate.
With the recent commercial introduction of the flat tension mask cathode ray tube, many process problems related to the curvature of the mask and panel have been alleviated or reduced. Necessarily, however, initial production of flat tension mask tubes has been based on continued use of the proven technology of mating a dedicated mask to a specific front glass panel throughout the manufacturing process. However, because the flat tension mask requires tension forces during the manufacturing process as well as after installation in a tube, somewhat cumbersome in-process support frames become necessary. These introduce complexity and expense in the manufacture of color cathode ray tubes of the tension mask type.
Thus, the desirability of simplifying the conventional production process remains as great as ever in the manufacture of cathode ray tubes of the flat tension mask type.
It has been recognized that color tube manufacture would be simplified if any mask could be registered with any screen (commonly termed an "interchangeable" mask), so that masks and screens would no longer have to be individually mated. Interrogation of the mask and screen to determine registration before assembly of the mask to the rest of the front panel assembly is, therefore, highly desirable.
U.S. Pat. No. 4,902,257; and its continuation-in-part, U.S. Pat. No. 4,973,280; describe and illustrate a variety of methods and apparati for registering a flat tension mask to a faceplate mounted screen with technology particularly adapted to achieve registration of interchangeable masks and faceplates. U.S. Ser. No. 07/710,738 discloses a complete system for registering the shadow mask to the screen and welding the mask to the faceplate.
The present invention follows many of the principles disclosed in these references and proceeds further bringing these technologies to commercial production. The complete manufacturing system disclosed by the above cited '738 application for mating a undedicated flat tension mask to a face panel mounted screen, includes: a) faceplate and mask loading, b) automatic shuttling of face panel and mask modules which carry the panel and mask alternately to a location beneath the assembly station, c) automatic lifting of faceplate and mask modules from the shuttle upwardly into the assembly station where they are registered with production accuracies, d) screen position interrogation with respect to the panel module, e) mask interrogation, and f) comparison of the tensed mask with respect to the panel module so that the location of the aperture array in the mask can be determined with respect to the actual location of the screen on the face panel in order to achieve true production registration consistency.
The term "registration" as used in these references and this application ultimately is used to describe the alignment of apertures in the mask with phosphor spots o the faceplate-carried screen, but to achieve that end it is necessary to properly align the panels and masks both with respect to their transport modules and the assembly station. The definition of registration herein, when indicated, encompasses these preliminary alignments as well.
In order to achieve production accuracies in an interchangeable panel-mask system, it is necessary to interrogate screen and mask locations for verification and registration control. According to the Ser. No. 710,738 preferred embodiment, these interrogation functions are provided by camera arrays carried on the mask and panel modules. The camera arrays thus travel with the modules from their respective loading stations to the assembly station, or alternatively, the interrogation camera arrays may be physically separate from the modules and moved into position, as needed, by transport means. The camera arrays must then be provided with positioning means to assure their constant placement. In the embodiments disclosed in the '738 application, the interrogation functions are provided by micro-television-type cameras.
Due to the very precise sub-mil positioning requirements of a CRT mask registration system, movement of interrogation cameras during interrogation must be kept to a minimum to allow for precise determination of planar coordinates. However, several widely spaced areas of the mask and panel must be viewed at high resolutions in order to determine registration. If one then uses multiple cameras to affect the proper range of fields to be interrogated at the proper resolution; one must overcome the problem of processing the information from multiple cameras in a small enough time period to make the interrogation system useful in a production environment.
Within the present interchangeable shadow mask environment, the present invention does this by utilizing only single bit binary values to represent pixels and then concurrently processing each camera's single bit values to evaluate the fields of view of the plurality of interrogation cameras simultaneously. By utilizing this approach great accuracy is attained at high production speed with minimal hardware. No reference or combination of references known to the applicant teaches these advantages.
Other references of interest may include: U.S. Pat. Nos: 4,525,735 to Krufka, 4,556,902 to Krufka, 4,665,429 to Krufka, 4,684,982 to Krufka, 4,717,955 to Krufka, 4,834,686 to Kautz, 4,980,570 to Yasunaga et al., 4,711,579 to Wilkinson, 4,989,082 to Hopkins,